It is perhaps the most important question of the BP oil spill -- but scientists don't appear close to answering it despite a historically vast effort.

In the 2 1/2 months since the spill began, the gulf has been examined by an armada of researchers -- from federal agencies, universities and nonprofit groups. They have brought back vivid snapshots of a sea under stress: sharks and other deep-water fish suddenly appearing near shore, oil-soaked marshes turning deathly brown, clouds of oil swirling in deep water.

But, with key gaps remaining in their data, there is wide disagreement about the big picture. Some researchers have concluded that the gulf is being spared an ecological disaster. Others think ecosystems that were already in trouble before the spill are now being pushed toward a brink.

"The distribution of the oil, it's bigger and uglier than we had hoped," said Roger Helm, a U.S. Fish and Wildlife Service official and the lead scientist studying the spill for the Interior Department. "The possibility of having significant changes in the food chain, over some period of time, is very real. The possibility of marshes disappearing . . . is very real."

Helm said that his prognosis for the spill had worsened in the past week -- as the amount of oily shoreline increased from Louisiana to Florida, despite cleanup efforts. "This just outstrips everybody's capability" to clean it up, he said.

This research has mainly occurred in the background, as public attention has focused on the "open-heart surgery" at BP's leaking wellhead.

The patient is a 600,000-square-mile sea, which contains swirling currents, sun-baked salt marshes and dark, cold canyons patrolled by sperm whales. Complicating matters is that even before the spill began in late April, the patient was already sick.

In recent years, Louisiana has been losing a football field's worth of its fertile marshes to erosion every 38 minutes. In the gulf itself, pollutants coming from the Mississippi's vast watershed helped feed a low-oxygen "dead zone" bigger than the entire Chesapeake Bay. Measuring the spill's damage, then, requires distinguishing it from the damage done by these other man-made problems.

So far, even the simplest-sounding attempts to measure the spill's impact have turned out to be complex.

The official toll of dead birds is about 1,200, a fraction of the 35,000 discovered after the Exxon Valdez spill in 1989. But this, too, has been called into question. Officials can only count the birds they can find, and many think a number of oily birds have sought refuge in the marshes.

"It's an instinctive response: They're hiding from predators while they recover," said Kerry St. Pé, head of a government program that oversees Louisiana's Barataria Bay marshes. "They plan to recover, of course, and they don't. They just die."